Question: Solve for $r$ : $-8 = -15 + r$
Answer: Add $15$ to both sides: $ \begin{eqnarray} \\ \hphantom{-8 {+ 15}} &\hphantom{=}& \hphantom{r} \\ -8 &=& -15 + r \\ \\ {+15} && {+15} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -8 {+ 15} &=& r \\ \end{eqnarray} $ Simplify. $ r = 7$